Turn the heater on to full hot, so coolant flows through the heater core. Top up the coolant level in the radiator and expansion tank, run the engine when the thermostat opens often air will burp out of the block of the engine run it until the electric fan comes on and goes of, let the engine cool down fully, top of the coolant run it again until the fan cycles on and off and let the engine cool down fully, recheck the coolant level do this until you know longer have to top up the coolant.